“Smart windows” regulate the amount of solar energy penetrating a room through the window, for the sake of comfort, or to provide savings in cooling and heating costs. This regulation can be actively controlled, such as in electrochromic windows.
Alternatively, the windows can be self-adaptive, and can have different optical properties in response to a change in the ambient temperature: Thermochromic windows exhibit a change in their color and absorbance. Thermotropic windows exhibit a change in their light scattering, and reduced transmission. The solar power transmission change has to be reversible, i.e. the materials or compositions should revert back to their original transmission when the temperature is reverted, in order for these materials to be applicable as smart windows.
Temperature responsive optical devices for providing light filtration are known in the art. Chalcogenide glassy semi-conductors such as those described in U.S. Pat. No. 3,790,250, for example, display temperature dependent light transmission characteristics.
Similarly, thermochromic materials are described in U.S. Pat. No. 9,128,307, and U.S. Pat. Publication 2013/0193386, and display temperature dependent color changes. These are based on materials which display a temperature dependent absorption. Such filtering devices are expensive for use in large area devices such as window panes. Furthermore, the filtration is achieved by absorption. Therefore, the filter itself, or the window itself, heats up, necessitating positioning of the filter away from the inside of the room or the temperature sensitive components.
Another design method of temperature responsive optical devices for providing light filtration is by reflection in multi-layer coatings having alternating low-high index of refraction, at high environmental temperature, such as those described in U.S. Pat. No. 9,128,307 and U.S. application 20060159874. These have usually high production costs due to the number of layers needed and have a physical problem since the reflection efficiency is dependent on the angle of impingement.
Another design method of temperature responsive optical devices for providing light filtration by reflection is presented by U.S. Pat. No. 4,261,331 where a transparent liquid is used as a temperature responsive optical devices for providing light filtration by scattering and reflection. This solution comprises a layer of a metal sulfate salt solution in thermal contact with a source of heat. The solution is characterized as having decreasing salt solubility with increasing temperature. When the heat source exceeds a predetermined temperature, the solution precipitates salt as a light scattering layer, substantially reflecting incident light in the solar spectrum. This solution uses a liquid layer, which is very troublesome in windows, needing special sealing techniques and complicated, costly production methods.
Another design method of temperature responsive optical devices for providing light filtration is via scattering, which is induced by refractive indices changes of a substance, relative to its host, as a function of temperature. An example of composition of blended polymers is described in “Thermotropic and Thermochromic Polymer Based Materials for Adaptive Solar Control”, A. Seeboth et. al, Materials 2010, 3, 5143-5168. It teaches blends, composed of two polymers, which are non-soluble in one another, and have different temperature dependencies of their refractive indices. Its main drawback is the thick polymer layer, in the order of a few millimeters, needed to obtain a significant filtration effect.
Another thermotropic apparatus is described in “Temperature Control of Light Transmission Using Mixed System of Silica Hollow Particles with Nanoparticle Shell and Organic Components”, M. Fujiwara et. al, ACS Appl. Mater. Interfaces 2015, 7, 1107-1113. Light transmission of a mixture of silica hollow particles with nanoparticle shell in various hydrocarbon solvents is varied with changes in temperature. The mechanism of the transparency variation is the adjustment of the refractive indices of the silica hollow particle and the organic components. As the refractive index alteration of the organic components with temperature is generally 10 times higher than that of silica, the difference in the refractive indices between them, induced by the temperature variation, changes the transparency of the mixture. However, the host described in this case is in liquid form, which makes it about impossible to be implemented in fenestration designs, where the liquid large thermal expansion and contraction can break the glass panes holding it, and the liquids may freeze into ice at cold temperatures. Moreover, the scattering substance is a hollow nanoparticle that may not be easily utilized with hosts that are not in liquid form.
Another design method of temperature responsive optical devices for providing light filtration is based on temperature-responsive hydrogels using poly(N-isopropylacrylmide) (PNIPAm), as described in “Temperature-responsive hydrogel with ultra-large solar modulation and high luminous transmission for ‘smart window’ applications”, Yang Zhou et. al., J. Mater. Chem. A, 2014, 2, 13550-13555. This hydrogel based device contains water in its liquid form, therefore good sealing is essential to prevent water evaporation that can hamper its operation. While the device described in the publication endured 20 cycles of measurement, this is not enough to indicate that the performance stability would be reliable in actual smart window applications.
In contrast, the present invention provides a composition useful as a coating or laminate for application upon windows or similar articles, to limit the transmission of solar energy entering into a dwelling, for climate control. The invention utilizes optical scattering and reflection, preferably using a single thin solid layer. The invention overcomes the disadvantages of the prior art, such as being independent of the angle of impingement, being non-absorbing and utilizing inexpensive materials and manufacturing methods. The invention provides over-temperature protection for a window having considerable solar exposure, such as on hot days. The composition of the invention allows transmission of the entire solar spectrum through the window on cold days.
The composition may be applied to windows that have already been installed, thus allowing retrofitting and upgrading of existing windows.
These and other advantages will be enlarged upon in the Summary and Detailed Description of the Invention that follow.